In mobile communication systems represented as cellular systems, a mobile device is required to determine which base station (cell) to connect to. This processing is typically performed on the mobile device side and is referred to as cell search. Generally, in the cell search, the mobile device detects a synchronization channel (such as a pilot signal) of a base station located within the receiving range, and attempts to synchronize with the base station for the detected signal. Once the synchronization is established, the mobile device transmits and receives control signals such as a system information to and from the base station and enters into the in-service area state in which the mobile device can communicate with the mobile communication system.
In practice, the mobile communication system may have a control over whether or not the mobile device should enter into the in-service area state, depending on the reception level or reception quality at the mobile device. Specifically, the mobile communication system dictates a threshold for entering into the in-service area state (a minimum required quality level) to the mobile device through the system information. Then, the mobile device enters into the in-service area state if a measurement detected in the cell search, such as the reception level or reception quality, is above the threshold. Otherwise, the mobile device reattempts the cell search.
Therefore, if the measurement detected in the cell search is below the threshold, the cell search and the reception of the system information are to be repeated. To restrain the increase in power consumption at the mobile device due to this repetition, conventionally, a determination has been made after the cell search and before the reception of the system information as to whether or not to receive the system information, depending on the reception level or reception quality at the mobile device. Specifically, if the measurement detected in the cell search is below a threshold for determining whether or not to receive the system information (a system information reception threshold), the mobile device reattempts the cell search without receiving the system information. This can reduce the frequency of receiving the system information in the cell search and thereby restrain the increase in the power consumption at the mobile device. Further, it can reduce the frequency of receiving the system information by setting the system information reception threshold higher than a default minimum required quality level, considering the factors such as the variation among signal measurements detected in the cell search.
However, in the conventional cell search control method, a problem exists in which the predetermined system information reception threshold of the mobile device becomes inappropriate when the mobile device moves to an area having a different minimum required quality level or when the base station has changed the minimum required quality level through the system information. For example, if the minimum required quality level becomes higher than the system information reception threshold, the frequency of receiving the system information increases excessively, leading to the increase in the power consumption. If the minimum required quality level is set lower than the original value and the system information reception threshold is fixed, the mobile device cannot update the minimum required quality level until the mobile device detects a measurement equal to or above the system information reception threshold. Even if the minimum required quality level of the mobile device is updated, the mobile device cannot enter into the in-service area state until the mobile device detects a measurement equal to or above the system information reception threshold, and thereby repeating the cell search.
In the situation where signals are received from multiple base stations, the threshold settings from the base stations may be different. For example, the mobile communication system may set the thresholds such that entering into the in-service area state is more difficult with one base station (cell 1) while it is easier with another base station (cell 2). In such situation, consider a mobile device with the minimum required quality level for the cell 1. Since the system information reception threshold is typically set higher than the minimum required quality level, the mobile device is to repeat the cell search operation without receiving the system information containing the minimum required quality level for the cell 2. As a result, it becomes difficult for such mobile device to reflect settings from the mobile communication system.